There have been numerious threads on this... The Bose (Clarion) headunit has differential outputs that need a VEN4 from http://www.peripheralelectronics.com This will give you front and rear RCA's for an entire system (I'm going from the VEN4 into a four channel amp that has a RCA outs for the sub amp - both amps have built in crossovers).

If you just want to add subs then grab the outputs for the 6x9's in the rear package shelf and use the VEN4 (Vendetta II may work). Either way, you'll need a crossover (built into the amp is fine).